A study of the photodesorption process for cryosorbed layers of H2, CH4, CO or CO2 at various temperatures between 3 and 68 K

The vacuum performance of the large hadron collider (LHC) at CERN will depend critically on the photodesorption of gas by synchrotron radiation and the re-adsorption of these molecules back onto the cold surface. The results of photon-induced molecular desorption by synchrotron radiation with a critical photon energy of 284 eV for H-2, CH4, CO or CO2 cryosorbed on a stainless steel surface are presented. Most measurements have been carried out in a temperature range from about 3 to 20 K. Measurements for CO2 were also performed at 68 K. The specific method used for this study has been to pre-deposit a known quantity of gas onto a cold surface, to irradiate the surface with a known photon dose and to measure the quantity of gas remaining on the cold surface by recording the pressure during warm-up. The average photodesorption yields of all gas species were found to increase with increasing surface coverage and to reach a saturation value. For Hz this value is approximately 0.5 for a coverage exceeding 10(17) molecules/cm(2), for CH4 and CO2 saturation occurs at about 0.5 molecules/photon at similar to 10(19) molecules/cm(2), while the corresponding final value for CO is about 0.04 molecules/photon at this coverage. (C) 1999 Elsevier Science Ltd. All rights reserved.